Many manufactured products are made by drawing sheet metal or molding a synthetic resinous material between a closely matched set of die surfaces. In order to carry out such metal drawing or molding operations, it is necessary to provide suitable tooling in the form of draw dies or mold sections. The part-shaping surface of such tools must be carefully made to precisely shape the workpiece to be drawn or molded into the three-dimensional configuration of the desired part.
Such tools have been used for many years. They are usually hard, massive bodies that are used in combination of two or more tool details (or simply details) in a powerful press. The press operates to separate the dies for the insertion of, e.g., a sheet metal blank. The press then closes and the blank is pushed by a convex tool (a male punch) into a concave cavity (a female punch). The flow of the metal is frequently controlled by a binder ring that operates in cooperation with the punches.
Such tools are typically made by casting a metal alloy of suitable hardness and durability into a configuration that duplicates as closely as possible the shape of the articles to be formed. Each of these castings may weigh hundreds or thousands of kilograms. Despite improvements in the casting of such tools, it has almost always been necessary to machine the cast metal forming surface of the tool to provide the desired surface shape and dimensions. The machining is required because the casting always undergoes shrinkage and distortion as it solidifies and cools to ambient temperature. Certain alloy compositions are known to undergo minimal shrinkage during the casting process. An example of such an alloy is Kirksite, a zinc-based alloy containing aluminum and copper of approximately ternary eutectic composition. However, even such low shrinkage, castable compositions still undergo some distortion from the intended configuration during the casting process and on cooling to room temperature.
It is known to design and construct patterns of the casting that are oversized to allow for shrinkage of the molten metal on solidification and cooling. It is also known to provide relatively hard mold surfaces to resist misshapen molds when the hot metal is poured. These practices have provided improvements in the accuracy of large castings, but have not solved the problem of producing a cast-to-size stamping die or mold tool.
The problem is further complicated by the fact that such tools comprise combinations of details that are intended to mate. However, each tool detail deforms in the casting process to exacerbate the problem. For example, many sets of sheet metal drawing dies comprise at least three members; for example, a male punch, a female punch and a binder ring. In general, the male and female punches have mating surfaces over and against which the sheet metal is stretched and drawn into the configuration of the desired part. The binder ring is shaped to fit about the male punch and to engage the sheet metal and control its flow as it is pushed into the female cavity.
In the case of the automobile industry, for example, scores of body parts such as door panels, fenders, hoods, roofs, trunk lids, body pillars and brackets are formed by sheet metal drawing or stamping operations, and each part requires a set of tools. The cost of such tools greatly increases when each member of the set must be cast and then machined to a final configuration. Such sheet metal drawing operations, plastic molding operations and the like would benefit greatly from a practice which would produce all pieces of a die set to substantially their final configuration in the as-cast condition.
It is an object of this invention to provide a method that combines both a sand mold preparation and casting practice by which a set of unmachined tool castings, e.g., a male punch, a female punch and a binder ring, are all brought into very close conformation with their desired shapes and dimensions. In other words, the complementary material-shaping surfaces of the two punches and the binder ring are cast such that when the castings reach room temperature, the working surfaces are in their desired finished conditions.
It is a further object of this invention to provide a method of preparing molds particularly suitable to the casting of the binder ring and, when necessary, the female punch so as to minimize distortion of these cored parts upon casting. It is a further object of this invention to provide a complementary practice of casting and handling the castings of such tool details that they interact with each other in the final shaping of each piece.
In accordance with the preferred embodiment of my invention, these and other objects and advantages are accomplished as follows.